It's not that serious.
John O'Shea will be a pretty good right back. He was missed last season as he was injured for almost the whole of the second half of the season. Rafael will be a decent back up, he should have matured a little. He is very good going forward.
I have written off G Neville, who is injured for the start of the season.
VDS has another season in him, and Kuszchak is a good backup. He's not world class, but I believe he is at least international class. He is at least as good as keepers like Howard, Friedal and Hart, if not in the class of Reina and Cech.
They still need to get a top class keeper, but it's not that urgent compared to a top centre back if Vidic leaves.